Three almost simultaneous attacks Wednesday on hotels managed by American companies in Jordan's capital carried the classic markings of terrorist hits by Al Qaeda, or its imitators. The bombings also shattered the long string of foiled plots by Jordan - a close Middle Eastern ally of the United States.

Shortly before 9 p.m local time on Wednesday, the Grand Hyatt, Radisson, and Days Inn hotels, all in the center of Amman, were hit by what the Jordanian police said were likely suicide bombers, leaving at least 57 dead and more than 300 wounded.

Jordan shares borders with both Saudi Arabia and Iraq, and is the native land of Abu Musab al-Zarqawi, the most wanted terrorist inside Iraq. Mr. Zarqawi, who spent time in Jordanian jails in the 1980s, has sought to strike at Jordanian targets in the past. Jordanian officials allege he was involved in a foiled New Year's Eve, 2000 attack that targeted the Radisson hotel and several tourist sites in Jordan.

Jordan's Deputy Prime Minister Marwan Muasher said in a CNN interview that Zarqawi was the country's "prime suspect."

Hundreds of militants from the country have poured into Iraq to fight alongside Zarqawi during the past two years, and security officials have long worried that militants - energized by the jihad in Iraq and with new skills - would come home to wreak havoc in the tiny kingdom.

The first known terrorist attack to have been carried out involving veterans of the Iraq jihad was in August of this year, when three rockets were fired at a US warship from Jordan's Red Sea port of Aqaba. The rockets missed their target and hit the Israeli port of Eliat. Jordan says militants operating out of Iraq were behind the attack, and Zarqawi's group later claimed responsibility.

"The Jordanians have been quite effective, their intelligence services are considered among the better in the region. Remember they've foiled a number of plots by Zarqawi's band,'' says Ralph Peters, a retired US Army intelligence officer who specialized in Islamic terrorist groups.

"But the terrorists are so determined. And the fact that the Jordanians are good could have pushed them all the farther underground, to lay low to take action like this. "

Mr. Peters says a successful attack in Jordan isn't as surprising as the fact it was so long in coming. He says Jordan is such a "crossroads" of Arab and Palestinian groups and populations that "you kind of expect that sooner or later someone decides to get tough or make some point."

If, as seems likely, the attack was carried out by Islamist militants, it will not be the first such attack in Jordan. US diplomat Lawrence Foley was gunned down in Amman in December 2002 in assassination that Jordan officials say was carried out by Al Qaeda and included the involvement of Zarqawi.

In April 2004, two Jordanians confessed on Jordanian television of plotting bomb and poison gas attacks on the US Embassy, the Jordanian prime minister's office, the Jordanian intelligence service, and other diplomatic missions. One of the Jordanians said he had met with Zarqawi in Iraq to plan the attack. He said Zarqawi gave him $170,000 to finance the operation, and that he used it to buy 20 tons of chemicals.

The country has long been a target of Islamist militants, particularly because of its close ties to the US and its peace deal with Israel.

"Jordan is clearly our longest, best, and most effective partner in the war on terror and it has been since the black September in 1970 when the Palestinian movement almost took over the kingdom," says John MacGaffin, the former associate deputy director for operations at CIA and a former senior adviser to the FBI. "If you ask bin Laden or anyone if Americans are the evil people in this existential battle, and ask who are the Americans' closest supporters in this battle, then it's clearly the Jordanians. And they happen to be Islamic, which makes it a whole lot worse."

President Bush condemned the bombings and offered US assistance in the investigation. "Jordan is a close friend of the United States, and we will offer every possible form of cooperation in investigating these attacks and assisting in efforts to bring these terrorists to justice," White House spokesman Scott McClellan said in a statement.

• Staff writers Howard LaFranchi in Washington and Alexandra Marks in New York contributed to this report.